Hill v. Florida is expected to hinge on the opinion of Justice Anthony Kennedy (Damian Dovarganes/AP)

It's about time.

Too late for those already dead, perhaps in time for those still living, the United States Supreme Court has moved at last to clarify the rules state officials must follow when determining whether capital defendants are "mentally retarded"* and thus precluded from execution under the Eighth Amendment. For over a decade, especially in the South, those rules have been manipulated by local officials and judges in ways that undermine the Court's 2002 landmark ruling in Atkins v. Virginia, which banned the execution of the mentally disabled—but permitted states to define for themselves that loaded term.

The justices will reassess this long-neglected area of capital law through a Florida case that illustrates marvelously the extent to which some states will go to execute condemned prisoners, even when those prisoners are manifestly retarded. The Court agreed on Monday to hear Hall v. Florida, a case brought by a condemned man, a convicted murderer, who was declared "retarded" by the Florida courts in 1992 and again in 1999, only to be declared "un-retarded" by the Florida courts in 2009. He claims this violates his constitutional rights. He's right—and the Court should say so.

Indeed, depending upon how the justices vote, Hall v. Florida could be the first step toward an important new constitutional standard for mentally disabled defendants in capital cases. The justices have an opportunity here to establish a universal benchmark that no state may avoid under the banner of federalism or the Tenth Amendment. They also have a chance to put some mettle into their existing precedent.

For what Hall v. Florida surely is—regardless of how the justices ultimately rule between now and the end of June—is an acknowledgment by the justices that their Atkins' "compromise," a landmark constitutional rule combined with a nod to states rights, simply doesn't work. When it comes to capital punishment, to the constitutional struggle between the rights of the condemned and the responsibilities of government officials, the Court simply can't rely any longer upon the tender mercies of those officials.

That it took the justices 11 years to reach this conclusion says as much about them as it does the lower court judges, prosecutors, and other officials who have doggedly sought since 2002 to execute men the Constitution says it would be "cruel and unusual punishment" to execute. Truth is, the problem with the Atkins' compromise was evident on the day it was announced. States can't kill the mentally retarded, Justice John Paul Stevens wrote for the majority, but states can define who is mentally retarded. So, logically, states simply changed their definitions, or applied warped new logic to old definitions, to execute those whom they had wanted to execute in the first place.

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Take Florida, for example, and the case of Freddy Lee Hall. Before Atkins, the Florida courts acknowledged that Hall was retarded—that he had been retarded his whole life—but state judges ordered him executed anyway because there was no constitutional rule precluding it. Then, after Atkins, when there was a constitutional rule precluding the execution of the mentally retarded, Florida ginned up a way to conclude that Hall wasn't mentally retarded after all—or at least not mentally retarded enough to spare him from execution. Here, exalting form over function to the bitter end, is Florida's brief asking the justices in Washington not to hear Hall's appeal.

The justices' concern about the constitutionality of these state laws—and the uneven way in which they are applied—sadly comes more than one year too late for Marvin Wilson, a man executed in Texas in 2012 despite the fact that he was patently retarded. It comes too late for John Ferguson, executed earlier this year in Florida despite his belief that he was the "Prince of God." But it may come in time for Warren Lee Hill, a condemned man in Georgia whom officials say they have a legal right to execute even though all of the doctors who have evaluated him agree that he is mentally retarded beyond a reasonable doubt.

States like Florida and Georgia and Texas are going to have to be restricted in how far they can go in defining down "mental retardation" in these capital cases.

The Hill case, like the Hall case, is all about states contorting themselves after Atkins to evade its reach. Georgia, for example, is the only state in the nation that still requires the condemned to bear the burden of proving his or her retardation beyond a reasonable doubt—the highest legal standard and one that doctors routinely decry as inconsistent with the vagueries of diagnosing mental illness or disability. Florida has adopted a bright line IQ test that similarly ignores the practical realities of evaluating men and women in this condition. The persistent application of these rules is inconsistent with Atkins, period.

In any event, as is the case with so many other disputes, Hall v. Florida is going to turn on Justice Anthony Kennedy, the lone conservative justice who sided with the majority in Atkins. On the one hand, Justice Kennedy is an avowed proponent of states right—in this case, the right of states to define for themselves the term "mentally retarded." On the other hand, Justice Kennedy is an avowed defender of the Eighth Amendment who put the "evolving" into its "evolving standards of decency."

If Atkins is going to mean anything after Hall, and if Justice Kennedy is going to stand by his choice in Atkins, then states like Florida and Georgia and Texas are going to have to be restricted in how far they can go in defining down "mental retardation" in these capital cases. The time has come to fix Atkins, to defend it, to strengthen it, so that men like Wilson and Ferguson and Hill and Hall can be permitted to spend the rest of their lives in prison without the shadow of execution hanging over their heads. The Eighth Amendment demands nothing less.

* While now outdated in common usage and considered offensive by many, the phrase "mental retardation" is a legal term of art still employed by the courts. I use it here only for ease of reference.